Significant amounts of gold occur in ores that are often referred to as refractory sulfides. The gold in these ores is often referred to as being refractory because it cannot be recovered by direct cyanidation. That is because the refractory gold is bound in the mineral lattice of the sulfides, and is therefore unavailable for recovery by traditional gold recovery techniques, such as direct cyanidation of the ore. Therefore, these refractory sulfide ores are commonly treated to chemically destroy the sulfide mineral lattice in which the gold is located and to release the gold for dissolution, such as by cyanidation, during subsequent gold recovery operations.
One technique for destroying the sulfide mineral lattice is to subject the ore to an oxidative treatment to oxidize sulfide sulfur, thereby releasing the gold for subsequent recovery. One method for oxidatively treating a refractory sulfide ore is pressure oxidation, in which a slurry of the ore is subjected to oxygen gas in an autoclave at elevated temperature and pressure to decompose the sulfide mineral, freeing the gold for subsequent recovery. Other oxidative treating methods include roasting and bio-oxidation of the ore in the presence of air or oxygen gas.
Treating whole ores by pressure oxidation or by oxidative roasting is expensive. Part of the expense is due to energy consumed in heating gold-barren gangue material in the whole ore, and especially the energy required to heat water in which the gangue material is slurried in the case of pressure oxidation. Also, process equipment for treating a whole ore must be sized to accommodate the throughput of gangue material, in addition to the throughput of refractroy auriferous sulfides, thereby significantly adding to the cost of process equipment. Moreover, side reactions may occur involving gangue material which can detrimentally affect the oxidative treating or can produce hazardous materials which require special handling.
One way to reduce the high energy and process equipment costs associated with oxidative treating of a whole ore, as well as the potential for problems associated with side reactions, would be to remove gangue material from the ore prior to the oxidative treatment. For example, one method that has been used to remove gangue material from refractory sulfide ores is flotation. In flotation, air is bubbled through a slurry of ore particles which have been treated with reagents and the particles of the ore which are less hydrophilic tend to attach to and rise with the air bubbles, thereby permitting separation of the ore into two fractions. Flotation has been used to prepare concentrates of sulfide minerals from refractory sulfide ores in an attempt to concentrate the gold into the flotation concentrate, thereby avoiding the expense of processing the gangue in subsequent oxidative treating. One problem with flotation of refractory sulfide ores, however, is that a significant amount of refractory gold often reports to the wrong flotation fraction, even when a high percentage of sulfides minerals is recovered in the concentrate.
There is a significant need for an improved method for processing refractory sulfide ores that avoids the high costs associated with oxidatively treating whole ores without the significant loss of refractory gold associated with concentrating sulfide minerals by conventional flotation.